Know your enemy

Abstract

In this article I examine the relevance of environmental control techniques in Malawi in the context of what we know about breeding, resting and feeding habits of the common Anopheles species. I hope that this article may help to put to rest some common misconceptions concerning Anopheles control as a malaria prevention strategy.

The common types of mosquito in Malawi

There are three main types (or genera) of mosquito in Malawi which bite people. Anopheles mosquitoes transmit malaria. They bite late at night (between 10 pm and 4 am) and are most abundant in rural areas. Culex mosquitoes are not of major public health significance in Malawi, although they may be involved in transmission of Wuchereria bancrofti which causes elephantiasis. They bite in the evening and during the night and are most common in urban areas. Culex accounts for more than 90% of all mosquito bites in urban areas. Aedes are the least common type of man-biting mosquito and bite during the day and evening. Aedes can transmit yellow fever and dengue fever but since these diseases are not common in Malawi they are of no major public health significance here. Aedes can be easily recognised by their black and white striped legs.

Anopheles species in Malawi

The principal malaria vectors in Malawi are Anopheles gambiae ss., An arabiensis and Anopheles funestus. These species are found throughout sub-Saharan Africa and we therefore know a lot about them not just from studies in Malawi but from many other countries in the region.

Mosquito breeding habits

Anopheles mosquitoes breed only in clean, sunlit water which is not organically polluted with faeces, rotting vegetation, garbage etc. They also like to breed in small natural (as opposed to man made) breeding sites. Common breeding sites include animal footprints, small areas of flooded grass, wet rice fields, borrow pits. Since almost all water in the immediate vicinity of human dwellings is polluted, Anopheles does not generally breed in this peri-domestic environment. For this reason cleaning or draining puddles around the house has no impact at all on Anopheles density or malaria. Because Anopheles needs clean water to breed in, their density is very low in urban areas and very high in rural areas. It is for this reason that malaria transmission is so much higher in rural areas compared to urban areas. Also, because Anopheles can breed in tiny amounts of water, as small as an animal's hoof print, it is generally impossible to control them in rural areas through destruction of breeding sites since the sites are simply too many and too dispersed. In some specific circumstances in urban areas where Anopheles breeding is restricted to a few small areas of urban cultivation (i.e. where surface water is not polluted) it may be feasible to reduce Anopheles density through destroying breeding sites or chemical larval control.

Unlike Anopheles, Culex loves organically polluted water. The preferred breeding sites include flooded pit latrines, septic tanks, soak-aways and blocked drains. Because there is so much polluted water in urban areas, Culex densities are correspondingly very high and Anopheles densities very low. Next time you are eaten alive on your “khonde” in Blantyre or Lilongwe at sundown, remember to blame Culex and not Anopheles.

Aedes mosquitoes are different again in their breeding site preference. They love to breed in the peri-domestic environment but not in large dirty puddles outside the house. They like a mixture of small natural and man-made sites. Natural sites include small amounts of water which collect in certain types of plant, including maize, and man made sites include discarded tyres, tins, jars, scrap metal containers and anything else that will hold a small amount of water.

Mosquito resting habits

To understand mosquito resting behaviour and its significance for control, it is necessary first to understand the stages of the life cycle during which resting takes place. After hatching from the pupa a newly emerged female will mate and almost immediately seek a blood meal. Anopheles will travel several kilometres, if need be, (though usually much less) to find a person to bite. Aedes and Culex tend to breed so close to human dwellings that they never have to fly more than a few hundred meters during their life. After feeding, the mosquitoes need to rest for about 3 days while the blood is digested and the eggs develop. When the eggs are matured they are laid in a suitable breeding site and the female will immediately seek another blood meal. Hence most mosquito resting time is after, not before, a blood meal. All three types of mosquito prefer to rest inside houses whilst their eggs develop. They seek out a quiet secluded place in the house (e.g. in the thatch, behind a curtain etc.) and wait until their eggs are mature. It is because Anopheles mosquitoes spend so much time resting in houses that indoor house spraying is one method used to control malaria vectors. However, promoting the cutting of grass and vegetation around the house, as a malaria intervention, is clearly not appropriate since Anopheles does not spend significant time resting in such areas and they can fly several kilometres to seek a blood meal. For how far should the grass be cut, several kilometres?

Summary

A knowledge of the biology of Anopheles helps us make rationale decisions about vector control through environmental modifications. Some rules of thumb for the entire sub-Saharan region are included below.

• Larval control of Anopheles is almost never feasible in rural areas and is only appropriate in select urban situations where breeding sites are few and easily destroyed. Mobilising communities to clean up their compounds may be justified on public health grounds BUT not specifically for malaria control.

• The cutting of vegetation around houses has no impact on malaria and there is published evidence dating back over years (Ribbands, 1946) to support this. Once again, some may feel that there is public health justification for promoting this behaviour to communities (e.g. reduce the proximity of snakes to houses) but it is important to understand that malaria transmission will remain entirely unaffected by such behaviour.